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# -*- coding: utf-8 -*-
# Copyright 2015 OpenMarket Ltd
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""
This module controls the reliability for application service transactions.
The nominal flow through this module looks like:
___________
\O/ --- event -->| | +--------------+
| - event ---->| event_pool|<-- poll 1/s for events ---| EventSorter |
/ \ ---- event ->|___________| +--------------+
USERS ____________________________|
| | |
V V V
ASa ASb ASc
[e,e] [e] [e,e,e]
|
V
-````````- +------------+
|````````|<--StoreTxn-|Transaction |
|Database| | Controller |---> SEND TO AS
`--------` +------------+
What happens on SEND TO AS depends on the state of the Application Service:
- If the AS is marked as DOWN, do nothing.
- If the AS is marked as UP, send the transaction.
* SUCCESS : Increment where the AS is up to txn-wise and nuke the txn
contents from the db.
* FAILURE : Marked AS as DOWN and start Recoverer.
Recoverer attempts to recover ASes who have died. The flow for this looks like:
,--------------------- backoff++ --------------.
V |
START ---> Wait exp ------> Get oldest txn ID from ----> FAILURE
backoff DB and try to send it
^ |__________
Mark AS as | V
UP & quit +---------- YES SUCCESS
| | |
NO <--- Have more txns? <------ Mark txn success & nuke -+
from db; incr AS pos.
Reset backoff.
This is all tied together by the AppServiceScheduler which DIs the required
components.
"""
class AppServiceScheduler(object):
""" Public facing API for this module. Does the required DI to tie the
components together. This also serves as the "event_pool", which in this
case is a simple array.
"""
def __init__(self, store, as_api, services):
self.app_services = services
self.event_pool = []
def create_recoverer(service):
return _Recoverer(store, as_api, service)
self.txn_ctrl = _TransactionController(store, as_api, create_recoverer)
self.event_sorter = _EventSorter(self, self.txn_ctrl, services)
def start(self):
self.event_sorter.start_polling()
def store_event(self, event): # event_pool
self.event_pool.append(event)
def drain_events(self): # event_pool
return self.event_pool
class AppServiceTransaction(object):
"""Represents an application service transaction."""
def __init__(self, service, id, events):
self.service = service
self.id = id
self.events = events
def send(self, as_api):
# TODO sends this transaction using this as_api
pass
def complete(self, store):
# TODO increment txn id on AS and nuke txn contents from db
pass
class _EventSorter(object):
def __init__(self, event_pool, txn_ctrl, services):
self.event_pool = event_pool
self.txn_ctrl = txn_ctrl
self.services = services
def start_polling(self):
events = self.event_pool.drain_events()
if events:
self._process(events)
# TODO repoll later on
def _process(self, events):
# TODO sort events
# TODO fe (AS, events) => poke transaction controller on_receive_events
pass
class _TransactionController(object):
def __init__(self, store, as_api, recoverer_fn):
self.store = store
self.as_api = as_api
self.recoverer_fn = recoverer_fn
def on_receive_events(self, service, events):
txn = self._store_txn(service, events)
if txn.send(self.as_api):
txn.complete(self.store)
else:
self._start_recoverer(service)
def _start_recoverer(self, service):
recoverer = self.recoverer_fn(service)
recoverer.recover()
def _store_txn(self, service, events):
pass # returns AppServiceTransaction
class _Recoverer(object):
def __init__(self, store, as_api, service):
self.store = store
self.as_api = as_api
self.service = service
self.backoff_counter = 1
def recover(self):
# TODO wait a bit
txn = self._get_oldest_txn()
if txn:
if txn.send(self.as_api):
txn.complete(self.store)
self.backoff_counter = 1
else:
self.backoff_counter += 1
self.recover(self.service)
return
else:
self._set_service_recovered(self.service)
def _set_service_recovered(self, service):
pass
def _get_oldest_txn(self):
pass # returns AppServiceTransaction
|
